In 1860 a naturalist named William Brewer set out to conduct the first geologic survey of the infant state of California. When Brewer arrived in the tiny adobe village of Los Angeles on December 2, he noted in his diary that “all that is wanted naturally to make it a paradise is water, more water.” Three weeks later a raging torrent of water—the worst rainstorm in 11 years—destroyed many of the adobes. Such is weather in California.

The ancient record, etched in tree rings, shows patterns similar to those of today: long dry spells punctuated by fleeting wet years. In the year 1130, the rain tapered off and did not start again in earnest for another 40 years. Multidecade droughts show up in tree rings throughout California's history.

The simple lack of rain that shows up in tree rings, though, is no longer a practical definition of drought. A better one is more subjective: the difference between the moisture we have and the moisture we need. By that standard, this current drought is unprecedented. Yes, California is drier than at any time since 1895, when people began recording the weather. But it is also unnaturally hot—2014 was nearly two degrees Fahrenheit warmer than the previous warmest year, and 2015 is shaping up to be even hotter—which cruelly boosts the land's need for water at just the moment when little is available. And human expectations for the land are unlike any in history. Almost 40 million people now call California home, and the rest of the country and much of the world depend on the food that grows there.

Californians can reel off the droughts they have endured: 1977, 1986–1991, 2001–2002, 2006–2007 and this one, which started in 2011. It is possible that future tree-ring scientists will see all of these not as a string of separate events but as the start of one of those medieval-style mega droughts; even those had wet years sprinkled within them. If the overcrowded salad bowl of California is indeed headed for decades of low precipitation in an era of unprecedented heat, the Golden State could end up being a very different place. In the worst case, it could be shorn of its lush agriculture and towering forests. In the best, its people could marshal the innovation for which they are famous and make their state the world's laboratory for water conservation and reuse. Either way, a painful adaptation to the new normal is under way.

To understand California's drought, you have to follow the water. This journey is full of surprises, starting with the fact that it begins some 6,000 miles away, out among the verdant western Pacific archipelagoes of Fiji, Vanuatu and the Solomon Islands.

Typically the sun warms the Pacific all along the equator, and the prevailing east-to-west surface winds push warm water into the island-rich sea west of the international date line. There the water literally piles up into a huge “mound” that is not only a few degrees warmer than the sea off the coast of South America but also about four feet higher. All of that heat fuels thunderstorms, which thrust moisture high into the atmosphere, where the jet stream—high-altitude winds that blow east instead of west—catches it for the trip to North America.

If the equatorial mound of hot water stays roughly west of the date line, we get La Nia, which is associated with droughts in the southwestern U.S. If the equatorial winds at the ocean's surface weaken or reverse and the mound slides east of the date line—and if the effect is sufficiently pronounced—we get El Nio, which brings additional rain to the West. What is happening now does not really resemble either El Nio or La Nia. That spot west of the date line during the past few winters was half a degree higher than the average 30 years ago, which is a lot in climate terms. It also got about a foot of extra rain in the winter of 2013–2014, as well as a Category 5 cyclone, which heaved a huge amount of heat from the unusually warm ocean high into the upper atmosphere. Two more gigantic cyclones in the region in early 2015 did likewise.

Scientists are loath to say exactly what causes what in a changing climate, but something about that warm-water mound in the western Pacific—perhaps combined with a shrinking differential between temperatures at the equator and the poles—seems to be jamming the meteorological gears. A “ridge” of high atmospheric pressure has parked itself over the eastern Pacific in the path of the moist jet stream, and like a boulder that has rolled into a creek, it is displacing the flow and pushing the jet stream north. What would have been California's water has been falling in enormous quantities onto Alaska and northwestern Canada, and it may have contributed to the historic snowfalls, from Chicago to Boston, this past winter and flooding in the U.K.

Such high-pressure jet stream–blocking ridges are common off the coast of California, but usually they dissipate within a few weeks, when storms break them apart. The current one has persisted since the winter of 2013–2014, diminishing only slightly from time to time and then, eerily, unusually, reassembling to ward off incoming moisture. Daniel Swain, a 25-year-old Ph.D. student at Stanford University, gave the anomaly the name that stuck: the Ridiculously Resilient Ridge, or Triple R. Several small storms punched through the Triple R this past winter—including a drenching rainstorm in February—but instead of dispersing, the ridge weirdly recoalesced. How long it will last, nobody can say.

Most of the water carried in from the western Pacific first touches ground high in the Sierra Nevada Mountains that run for 400 miles along the state's eastern border. I began searching for California's missing water there—specifically, at Echo Lake, high above Lake Tahoe. In wet years, the water blows in on the jet stream and falls here in titanic quantities. A man I know once skied into the area to find his cabin on the lakeshore, dug holes in the snow all over the side of the mountain to look for the house and never found it; he had to ski out in the dark. At Echo Lake this year, in contrast, almost no snow fell. In the lee of the Triple R, the winter of 2013–2014 saw historically low levels of Sierra Nevada snowpack, and this past winter was even worse—at only 5 percent of the average. In April the drifts at Echo Lake are usually human-high, but when I arrived on Tax Day, only a few tiny scraps of white huddled underneath the trees.

From Echo Lake, I drove 200 miles south, passing Yosemite National Park, to visit Nathan Stephenson, a plant ecologist for the U.S. Geological Survey who works among California's marquee giant trees in Sequoia National Park. Sequoia looms above the Central Valley's Tulare County, ground zero of the drought. The Kaweah River flows out of the park, down to the much depleted Lake Kaweah and then on down to the valley. Stephenson has seen droughts come and go in his 35 years in the park but never like this one. “I'd estimate from eyeballing that a third of the oaks on the slopes are dead or dying,” he said, scanning a mountainside forest that, even to the untrained eye, looked pale and tired, dotted with brown trees. Stephenson is tall and lanky, with a gray beard and the sunny disposition of a man who gets paid to hang out in a national park. But he was morose as he gazed at the hillside from under the bill of his usgs hat. “This is only April,” he marveled.

We climbed back into his Subaru and drove up the mountain to a copse of incense cedars riddled with specimens starkly golden-brown. “They're hundreds of years old and very resilient—hosts to few insects,” Stephenson said. “We've jokingly called them ‘the immortals’ because they never seem to die.” He paused and put a hand out to feel a cedar's brown needles. “I guess they're mortal now.” Finally, we ascended to the kingdom of the eponymous sequoias themselves, many of which stood amid heaps of their own dead needles—testimony to the way the drought was nibbling at their extremities.

The usgs has been tracking 20,000 trees of various types in 30 widely spaced plots here for as long as 33 years. The trees in them, sequoias included, are dying in ways both predictable and otherwise. In normal times, an unbroken thread of water extends from a tree's roots to every leaf or needle, siphoned up through tiny capillaries as the tree transpires water into the air. Now, though, trees of all types are dying of cavitation: the thread of water breaks, air bubbles get into the capillaries and that is that. Other trees slam shut the pores on their leaves during dry periods to retain water. But then they cannot breathe carbon dioxide. Usually moisture returns and the pores reopen before the trees asphyxiate, but this drought has been so long, dry and hot that many trees are fatally squeezed between holding onto their water and breathing. And then there are the beetles, attracted to drought-stressed trees, that are devastating enormous stands of pine throughout the West. Once a tree dies, the beetles fly off to the next. Sometimes, during this drought, they fly in swarms so intense that you can scoop them out of the air with a baseball cap. This past spring an aerial survey of a huge swath of California's Sierra Nevada forests, including Sequoia park, found more than 10 million dead trees—10 percent of the trees in the surveyed area, most of them killed in the previous year. If the drought continues long enough, it could sear the majestic forests off of California's high ground and annihilate the giant sequoias that include the General Sherman tree, at 275 feet in height and 37 feet across at the base, the biggest in the world, by volume.

A massive die-off would be an enormous loss for the state, but it could be dire for the planet, not only because it would release untold tons of carbon dioxide into an already warming atmosphere. Last year Stephenson was lead author of a massive study—of 673,046 trees of 403 species across six continents—which shocked the botanical community by finding that, contrary to popular belief, trees grow faster the bigger and older they get. If the Sierra Nevada forest continues its die-off, it will be repopulated by a very young forest, which might suck less climate-warming carbon dioxide from the atmosphere than the current, multiage one.

In wet years, the snow that piles up in the Sierra Nevada contains enough water to fill the state's reservoirs. Down the western slope it trickles each spring and summer, and left to its own devices, it finds its way into the next stop on our pursuit of California's absentee water—a gigantic feature of this crowded state that hides in plain sight: the 1,100-square-mile Sacramento–San Joaquin River Delta.

The delta lies just east of San Francisco Bay. Before the arrival of settlers, it was a freshwater marsh of channels and sloughs and islands, but it is now mostly planted in crops and even hosts more than half a million people in such cities as Antioch and Rio Vista. But huge tracts remain undeveloped floodplain—spooky, jungly, dead-flat wilderness that the madding crowd hardly ever sees—shot through with about 700 miles of tangled waterways. This is the biggest estuary on the West Coast, the confluence of the rivers that drain the vast Sacramento and San Joaquin valleys, and it is the grand clearinghouse for California's managed surface water. Water released from northern reservoirs to southern farms and cities must pass through here. It took hours of crisscrossing dirt roads and causeways to find the little corner of the delta known as the Clifton Court Forebay, where loom the pump houses that push water down the wide, concrete-lined trench that is the California Aqueduct toward Los Angeles, 340 miles away, and through the Delta-Mendota Canal to the sprawling farms of the Central Valley.

California's surface water is so heavily managed that it seems more an industrial product than a natural resource. A network of state and federal reservoirs; a complex grid of canals and aqueducts; and a dizzying tangle of water laws, water rights, environmental regulations, court orders and legal opinions divide water up in ways guaranteed to infuriate everybody. About half the surface water is left in the streams, rivers and delta to maintain wetlands and fish habitat, to comply with the Endangered Species Act, and to keep saltwater from backing up through the delta and flowing into the canals and aqueduct.

The remaining half of California's surface water is allocated to humans: 20 percent to the cities—which in April were ordered by Governor Jerry Brown to reduce consumption by, on average, a quarter—and 80 percent to farmers. In theory. This year and last, surface water has been in such short supply that most farmers were allocated zero.

Given the micromanagement of California's surface water, it is shocking that the taking of groundwater, by far the majority of the state's water, is almost completely unregulated. California is the only state where you can pump as much groundwater as you like as long as you do not waste or sell it. The current drought has set off a kind of arms race in the Central Valley, with every farmer eager to go deeper than his or her neighbor, “like a bunch of four-year-olds with one milk shake and lots of straws,” in the words of one agricultural economist. Nobody knows how much is being pumped out, but groundwater levels are historically low. The farmer with the deepest well in a given area draws down the water, and if that means the neighbors' wells go dry, so be it.

Some are going as deep as 1,500 feet to reach water that may have rained 10,000 years ago. Such “fossil” water, in contact with geologic substrata for that long, is frequently foul with arsenic, chromium, salt and other contaminants. Drilling that deep is also expensive. Farmers who can find a driller to do the job—waiting lists are a year long—might spend half a million dollars on the project, and that does not include the high cost of pumping the water to the surface from such abysmal depths.

About 190 miles south of the delta one afternoon, near the farm town of Visalia, I followed a plume of smoke to a field full of dead orange trees that had been bulldozed into piles the size of large houses and set alight. The owner, who stood watching gloomily, told me he had leased the 80 acres and its 10,600 healthy trees to a tenant farmer, who last spring had hooked up illegal pipes and sold the farm's well water to a neighbor, letting the trees die.

It is not just landowners who are getting hurt. Yolanda Serrato in East Porterville, Calif., a poor, unincorporated farmworker town in Tulare County, was watering her small lawn last December when the hose sputtered and the water stopped—for good. The shallow wells of about 400 of her neighbors went dry around the same time, leaving them dependent on a hodgepodge of public assistance and charity. When I met Serrato, she was leaning on her chain-link fence and peering down the street for the pickup truck that she hoped would bring her a few bottles of water. It was hard not to see East Porterville as a harbinger of a day when Californians are forced from their homes by a lack of water.

The first law of hydrodynamics is that water flows toward money. It is likely to be a long time before most Californians, especially in the coastal cities, confront dry taps. San Francisco draws its water from the pristine Hetch Hetchy Reservoir, 167 miles away in Yosemite. Los Angeles—as anybody who has seen the movie Chinatown knows—dried out the Owens Valley, more than 200 miles away, in the 1920s and now gets most of its water from reservoirs even farther north. As long as California has even a drop of water, it will doubtless run toward wealthy coastal residents.

In the Central Valley, however, the trouble is only beginning. To understand why, we have to follow California's water deep underground. The Central Valley is essentially a 20,000-square-mile trough of layered clay, gravel, silt and sand, wedged between mountain ranges of hard rock. Water travels laterally in layers of gravel and sand with ease, which is why a farmer pumping groundwater can suck water away from a neighbor. Moisture is primarily stored, though, in layers of clay, which drip their load slowly into the gravel and sand. It is the way in which clay stores water that makes the current pumping frenzy so worrisome.

Disasters have a way of catapulting scientists from obscurity to fame overnight. Michelle Sneed, a young usgs geologist, toiled for years to become an expert in a dull field—ground subsidence—that has suddenly become crucial to the state's future. With startlingly direct blue eyes and long, wavy hair, she seemed to be enjoying her moment as a scientific rock star. As we sat in her office in Sacramento, at the northeastern edge of the delta, she turned her palms up, intertwined her fingers and explained that the microscopic structure of clay consists of tiny plates cocked haphazardly. “Imagine how much water you could fit into your kitchen sink if you threw in a bunch of dinner plates and left them leaning on each other every which way,” she said. Then she pivoted her hands to press the palms together. “Now imagine stacking those plates neatly and what that would do to the space for water between them.” That is essentially what happens when too much water is pumped out of the ground too quickly; the microscopic plates in the clay slide into a stacked-up position. The clay layer, in other words, collapses.

Hundreds of feet above, the ground collapses with it. Vast areas of the Central Valley have subsided, since the 1920s, by nearly 30 feet. In just two years—between 2008 and 2010—more than a tenth of the Central Valley sank two inches. That makes work for road crews who fix cracked highways and bridges and for railroad workers who relevel track. It also complicates the delivery of water around the state. Canals and aqueducts can run for hundreds of miles without pumps because they slope ever so slightly downhill. It does not take much subsidence to interfere with the flow, which is what happened last year, among other places, at the spot where a big canal meets the San Luis Reservoir in central California. But the interruptions to water delivery are hardly the worst of it. Once subterranean clay collapses, it can never again store water. So California's pump-frenzied farmers are not only depleting the aquifer on which they depend, they are also destroying it.

The only hope is to recharge what is left of the aquifer as fast as possible. The problem is that not all ground is equally rechargeable. Underneath about half the Central Valley is Corcoran clay, the remains of an ancient lake bed, which can be punctured by wells but which, unlike most clay, remains largely impermeable to water. Geologists can identify areas with no Corcoran clay—areas that are permeable and thus geologically suitable to flood for groundwater recharge. But some are covered with subdivisions, shopping centers or farms; identifying permeable ground and getting permission to flood it is a formidable task.

Scientists at the University of California, Davis, are running an experiment with the Almond Board of California to see if almond orchards that sit above geologically appropriate soil can be flooded, when the trees are dormant in winter, to recharge the aquifer. That raises not only geologic questions but also legal ones: California law requires farmers to use the water that they receive from the state for “beneficial uses” only, and recharging groundwater might legally be banned as “overwatering.” Then there is the question of whether a farmer who banks water this way has a claim to receive an equal amount later. And to flood a field or grove to recharge groundwater, it takes more than permission and legal rights; it takes water. Lately there is not enough water to nourish today's crops, let alone enough to bank for future ones. Any massive recharge scheme will have to wait for a wet year.

The crisis has been severe enough to give Governor Brown and the legislature cover to change California's 150-year-old water laws in a baby step toward regulating groundwater. Under a law passed last November, local water agencies in each of the state's 515 distinct groundwater basins will have five years to come up with plans for sustainable use and 25 more to achieve them. That is going to shake up the state politically because city water departments, farmer-run irrigation districts, county water commissions and other water-management agencies—all of which live in their own worlds, with their own proprietary data and competing interests—are going to have join into groundwater sustainability agencies, or GSAs, to share their most valuable resource. In a cheaply paneled temporary office building that serves as the office of the city of Tulare Water District, halfway down the San Joaquin Valley, I met a young man named Benjamin Siegel who has been assigned the thankless task creating a GSA with the city of Visalia and a local irrigation district. “It's like writing a new language,” he said.

Fifteen miles up the road Denise Atkins, the county's administrative analyst for water resources, told me that just getting everybody to agree on who will have a voice in the local GSA is a nightmare, let alone getting people to agree to share data. “Five years ago if you wanted to ask a grower, ‘How do you feel about a meter on your well?’ you'd better wear Kevlar,” she said. “Now farmers are getting enthusiastic about knowing how much water they use.” She leaned across her cluttered desk, rolled her eyes and added, sotto voce, “Though it's usually, ‘My neighbor is pumping too much.’”

Scientists differ on the explosive question of whether the drought is caused by anthropogenic climate change. The National Oceanic and Atmospheric Administration said no last year, the Intergovernmental Panel on Climate Change said maybe and a team of Stanford climate scientists that included Swain—christener of the Triple R—said yes. The Stanford crew modeled current and preindustrial climates and determined that the conditions associated with the Triple R are three times as likely now. But whether or not climate change is causing the drought, everybody seems to agree that the additional heat is exacerbating the effects of low moisture, from the forests of the Sierra Nevada to the farms of the Central Valley.

After years of something that looked kind of like La Nia, in March, noaa declared the start of a weak El Nio but cautioned that it probably will not affect weather much in California anytime soon. California might have some wet years in its near future, but the soil from the top of the Sierra Nevada to the bottom of the Central Valley is so desiccated that it will take years to properly hydrate the ground and much longer to begin recharging the groundwater. The state can choose to view current conditions as an anomaly and “manage this as a disaster,” but that would be a terrible mistake, said Noah Diffenbaugh, a senior fellow at the Woods Institute for the Environment at Stanford. “It's clear that California is in a different climate now.”

If that different climate involves, say, a 30-year drought akin to the ones in the Middle Ages, the mountain forests will die off because their water is not managed, and the next casualty will be the farms and orchards of the Central Valley that have been so emblematic of California for the past century.

One line of thought about the end of California's agriculture goes like this: So what? Agriculture constitutes only about 2 percent of California's economy, and the flood of inexpensive, water-intensive food that the world has enjoyed was perhaps always the unrealistic illusion of people without a millennium-long perspective. “California would be fine” without agriculture, Richard Howitt, a dry-witted, British-born agricultural economist at U.C. Davis, told me. “We'd turn into an Arizona economy. We'd phase out irrigated ag and move to movies [2.1 percent of California's gross domestic product], information technology [8 percent] and everything else.” Fruit, nuts and vegetables would doubtless become more expensive for everybody, but California itself could easily survive on the manufacturing, health care, finance and education that make its economy the seventh largest in the world—especially if it was not diverting four fifths of its usable water to irrigated agriculture.

Realistically, though, it is hard to imagine a state as innovative as California simply allowing the pride of its fields to disappear. More than a third of the Central Valley's agriculture is in grapes and tree crops—almonds, walnuts, pistachios, citrus—that represent an enormous investment than can take as long as seven years after planting to pay off. Farmers are already turning to a vigorous high-tech industry that makes GPS-equipped irrigators, weather-based irrigation, soil-moisture sensors and other agroelectronics designed to reduce water use. Even more radically, in June the state took the unthinkable step of placing water restrictions on California's agricultural royalty—those who hold Gold Rush–era riparian water rights in the Sacramento and San Joaquin valleys that have long been considered inviolate. It is easy to get the sense, traveling around California, that the pain—but also the creative thinking—has only begun.

The drought is transforming California in almost every conceivable way—meteorologically, geologically, biologically, agriculturally, socially, economically and politically. The combination of low moisture and high temperature most likely will be the condition of the future. Even when sporadic wet years occur, the inexorably warming climate assures that precipitation will fall not as heavy snowpack that parcels out water slowly but as crashing torrents of rain. That is why last November Californians voted for Proposition 1—more than $7 billion for water infrastructure, almost half of which will go to building new dams and reservoirs—a public works project of massive proportions. And therein lies the silver lining in the California drought: one person's cost is another's opportunity.

The Army Corps of Engineers wants to pull the concrete out of an 11-mile stretch of the Los Angeles River—now an ugly storm drain that does little but funnel as much as 207 million gallons of water a day into the ocean. The project would allow at least some of that water to recharge the aquifer and inject more than $1 billion into the local economy.

Desalination has the potential to provide the coasts with nearly limitless water, but it is wildly expensive, has an enormous carbon footprint because it takes so much energy and generates oceans of intensely salty brine that are hard to dispose of safely. The real potential for drought management is in water conservation and recycling. The Pacific Institute, an environmental think tank in Oakland, estimates that just getting homeowners to use water more efficiently indoors and out could save California about three million acre-feet a year—close to a third of its urban water use.

Proposition 1 includes $725 million for water recycling—seven times more than the state has ever devoted to it. That is only about a fifth of what the Californian branch of the WateReuse Association, the trade group for the water-recycling industry, thinks it would take to maximize the potential for recycling in the state, but the state money is intended to attract city, county and private money to water reuse projects. Retrofitting city parks, golf courses, factories, office buildings and even homes with “purple pipe”—which carries water clean enough for landscaping, toilets and other nonpotable uses—is about to become a multimillion-dollar sector of the economy.

The transition has already started in Orange County, which since 2008 has been treating more than a third of its wastewater to potable standards and injecting it into the aquifer. The county makes another 17 percent of its wastewater clean enough for industrial processes, landscaping and such domestic uses as flushing toilets. The infrastructure was expensive, but most of the treated water costs the county a little more than half of what it would cost to import water from the rapidly depleting Colorado River. Last November, San Diego's city council voted to spend almost $3 billion on the equipment that will allow the city to recycle enough water for a third of its citizens. WateReuse insists that purifying wastewater could supply all the municipal needs of eight million people—a fifth of California's population—and create untold jobs in the bargain.

The new normal is a little frightening, but this is California. Problems, yes, but there's gold in them thar solutions.